In conventional modems, the clock recovery network has been designed to operate at high frequencies, e.g., above 10 MHz, or low frequencies. The high frequency clock recovery networks have proven quite acceptable but typically utilize delay elements. To operate these high frequency clock recovery circuits at lower frequencies requires that larger delay elements be utilized, and below approximately 10 MHz the required size of the delay element becomes unacceptable. For this reason, separate clock recovery networks have been designed for low frequency systems, these low frequency clock recovery networks typically incorporating phase lock loops with multiple control elements whose repeated adjustments become tedious and are often impractical.
In order to overcome this difficulty, I have invented a clock recovery network which is effectively useful at both low and high frequencies, and I have disclosed such a clock recovery network in my co-pending and concurrently filed application Ser. No. 79,654 entitled Clock Recovery For QPSK Modems and assigned to the same assignee as the present application. This co-pending application is incorporated herein by reference as if fully and completely repeated hereinbelow.
The present invention is an improvement in the clock recovery network disclosed in my co-pending application, and is directed to the use of such a clock recovery network in a universal modem. In present modems, the clock recovery network is designed to operate only at a single frequency, and in order to accomodate different bit rates, the clock recovery network must be replaced. Even in the clock recovery network disclosed in my co-pending application, the Synchronized Voltage Control Oscillator (SVCO) is tuned to a center frequency in the vicinity of the clock frequency to be recovered, and the incoming data stream is then used to positively synchronize the oscillator with the incoming data. If a different bit rate is to be employed, the oscillator would have to be replaced with a different oscillator tuned to a different center frequency. This requirement of replacing the clock recovery network in order to accomodate different bit rates can be quite troublesome in a universal modem which may be operated frequently at different bit rates.